Japanese patent publication 2001-22179A (U.S. Pat. No. 6,336,014) discloses a side seal member that is provided between an end part of a rotatable developing roller and a developing unit housing and that is located adjacent to a developer supply port of the developing unit housing. The side sealing member includes an upstream side base, a downstream side base and a felt member adhered to the upper surfaces of the upstream and downstream bases.
Because fibers of the felt member are not unidirectional, developer entering the felt member may be moved in a direction away from the supply port by sliding contact between the felt member and the developing roller to cause toner leakage.